1. Field of the lnvention
The present invention relates to silica coatings, articles, such as optical devices, bearing silica coatings thereon, and to processes for preparing such articles.
2. Background Information
Improvement in the transmittance of light through optical devices such as windows, solar collector cover plates, lenses and prisms has long been sought so as to increase their usefulness. Optical devices having surfaces coated with antireflecting layers, typically having an optical thickness of one quarter of a wavelength, are known. Also known are optical devices in which surface reflections are reduced by altering the surface to provide a gradient index of refraction between that of the medium traversed by the incident light, such as air and that of the body of the optical device.
One method for providing such an altered surface is disclosed in Great Britain Patent No. 29,561. It involves tarnishing glass surfaces in aqueous solutions of sulphuretted hydrogen in order to reduce the reflection of light therefrom. Such a method is not useful for producing an antireflection surface on polymeric substrates.
Another method for providing such an altered surface is disclosed in Nicoll (U.S. Pat. No. 2,445,238). This patent discloses a method for reducing reflection from glass surfaces in which the glass is heated in a vapor of hydrofluoric acid to form a skeletonized surface. Such skeletonized surfaces are difficult to reproduce and maintain.
Moulton (U.S. Pat. No. 2,432,484) discloses a technique for forming a non-gelling, nonuniformly dispersed layer of anhydrous colloidal particles on the surface of articles. The particles form a random arrangement of peaks on the article surface to provide antireflection characteristics.
Moulton (U.S. Pat. Nos. 2,536,764 and 2,601,123) discloses a transparent binder coating prepared using a dilute solution of tetraethylorthosilicate in organic solvent to render the colloidal particulate layer taught in the '484 patent resistant to wiping and handling, as such layers are inherently readily susceptable to injury.
Geffcken et al. (U.S. Pat. No. 2,366,516) disclose an antireflection layer formed by applying an aqueous dispersion of a gel-like low-hydrated oxide, such as silicon dioxide, to an object and heating the coated object to a temperature of 250.degree. C. to form a hardened layer. Such a layer cannot be applied to most polymer substrates due to degradation of the substrate by heating to 250.degree. C.
Baker et al. (U.S. Pat. No. 3,301,701) disclose rendering a glass base antireflective by coating with a finely divided silica substantially free of silica gel. Such a coating would be expected to be brittle, weak, and powdery.
Land et al. (U.S. Pat. No. 3,833,368) disclose antireflection coatings for photographic products which are an eighth-wave layer of a fluorinated polymer applied over an eighth-wave layer of silica, the silica layer having been formed from an aqueous colloidal silica sol.
Swerdlow (U.S. Pat. No. 4,409,285) discloses an antireflection coating for optical surfaces, the coating formed from silica and/or alumina particles in a polymeric binder with particles protruding from the surface of the binder. 20 to 98 weight percent of the particles have a size in the range of 7 to 50 nanometers (nm) and 5 to 65 weight percent of the particles have a size in the range of 75 to 150 nm.
Yoldas (U.S. Pat. Nos. 4,271,210 and 4,346,131) and McCollister et al. (U.S. Pat. No. 4,273,826) disclose anti-reflection coatings produced by coating a substrate with a metallo-organic compound, e.g. alkoxide, and heating the coated substrate at temperatures which decompose the organic components of the coating leaving a metal oxide layer on the substrate. The temperatures necessary to decompose the organic components would also decompose polymeric substrates.
Dorer et al. (U.S. Pat. No. 4,190,321) disclose an antireflective coating of a metal oxide in the form of discrete leaflets of varying heights and shapes. This coating is susceptible to damage during handling due to the fragility of the leaflet structure.
Cathro et al, (Silica Low-Reflection Coatings for Collector Covers, by a Dip-Coating Process, SOLAR ENERGY, Vol. 32, No. 5, 1984, pp. 573-579) disclose low-reflection silica coatings prepared from ethanol-based silica sols which are aged at pH 7. Aging causes an increase in optical density and viscosity due to the agglomeration of silica particles prior to coating. Although good adhesion of the coating to glass is said to be obtained by heating at elevated temperatures, adhesion to polymeric substrates is poor, i.e., the coating can be wiped from the surface of the substrate by rubbing with a tissue.